Ships vanish from AIS due to range limits, terrain, lack of carriage, permitted switch-off, malfunction, or bad sensor data.
A typical ship-to-ship AIS range at sea is stated by IMO as 20 to 30 nautical miles depending on antenna height, but terrain, stations and radio conditions prevent a guaranteed universal range. Not all ships carry AIS, fitted units may be switched off in limited safety or security circumstances, and received data may be incomplete, unavailable or wrong.
The built-in integrity test can report equipment malfunction but does not validate the accuracy of connected sensor data; routine checks and corroboration remain necessary. AIS does not prevent collisions by itself and does not replace visual lookout, radar plotting, use of all available means or compliance with the COLREG. A missing report does not identify the cause or make the vessel itself disappear.
